Israeli Prime Minister Benjamin Netanyahu's former chief of staff is set to give evidence against his old boss, prosecutors have announced.

Ari Harow has agree to be a state witness in two cases accusing Netanyahu of bribery, fraud and breach of trust, it has been reported.

Israeli newspaper Haaretz reports: 'Prime Minister Benjamin Netanyahu's former chief of staff Ari Harow has reached an agreement with the prosecution to turn state's witness in two corruption cases against Netanyahu.'

Israeli Prime Minister Benjamin Netanyahu gestures to the press in Jerusalem on January 14, 2016

And public radio stated: 'Former chief of staff Ari Harow signed today a state's witness agreement.'

There was no official comment on the reports and any leaked details of the case are banned from publication by court order.

US-born Israeli immigrant Harow worked for Netanyahu from 2009-10 and 2014-15, when he stepped down over allegations of corruption.

He has been under investigation for more than two years on suspicion of bribery, breach of trust, conflict of interest and fraud, the media said.

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Haaretz said Harow has already been giving investigators information on two of the ongoing investigations into Netanyahu.

One is based on suspicions that the premier unlawfully received gifts from wealthy supporters, including Australian billionaire James Packer and Hollywood producer Arnon Milchan.

Also being probed is a suspicion that Netanyahu sought a secret deal with the publisher of top-selling daily Yediot Aharonot.

The proposed deal, which is not believed to have been finalised, would have seen Netanyahu receive positive coverage in return for him helping scale down the operations of Israel Hayom, Yediot's main competitor.

The investigations have stirred Israeli politics and led to speculation over whether Netanyahu will eventually be forced to step down.